If you do most of your woodworking measurements with a plain
old ruler, you should run right out and get one of these tools.
It's called a combination square.

Combination Square

This tool has a ruler built into it,
but it also has a sliding block that can hold the ruler
at 90 or 45 degrees to the edge of a piece of wood.
The perpendicular part comes in really handy,
but the fact that it slides also enables you to easily
replicate measurements.
You just set the ruler for a particular distance,
lock the setting using the thumb knob,
and make as many marks at that distance as you need.
SOme of these have additional blocks that let you set
an aritrary angle, or mark the center of a corner or circle.
You can also use it as an impromptu depth gauge.
This one comes with a built-in bubble level,
which could come in handy,
though I have better levels.
Most come with a small metal scribe for marking.
I never use that, either.

It's important that you get a good accurate one of these,
and that you take care of it. The accuracy of all your work
will come to depnd on this tool, so don't mistreat it.